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Citrate-Ascorbate as a Highly Selective Extractant for Poorly Crystalline Iron Oxides

Departmento de Ciencias y Recursos Agrícolas y Forestales, Universidad de Córdoba, Apdo. 3048, 14080 Córdoba, Spain

* Corresponding author ( crltocaj{at}uco.es).

ABSTRACT

Existing methods for dissolving poorly crytalline Fe oxides (primarily ferrihydrite) in soils and mineral mixtures are simple and rapid but often lack selectivity. In this work, we tested a more selective, alternative ascorbate-based extractant. Ascorbate combined with citrate was found to be highly effective in dissolving poorly crystalline Fe oxides while dissolving little goethite and hematite. Citrate-ascorbate and oxalate extracted similar amounts of Fe from most of the soils and Fe-rich materials studied. However, citrate-ascorbate was more selective than oxalate, since it dissolved only negligible amounts of allophane or imogolite and magnetite. The recommended citrate-ascorbate extraction procedure (0.2 M sodium citrate-0.05 M sodium ascorbate, pH 6, 16 h) is simple to implement and relatively inexpensive, and uses nontoxic chemicals. Thus, it can be used on a routine basis for estimating poorly crystalline Fe oxides in soils. By subtracting citrate-extractable Fe and Al from citrate-ascorbate-extractable Fe and Al values, respectively, one can estimate Fe and Al contained in poorly crystalline Fe oxides. The citrate-ascorbate extraction, combined with others (oxalate, Tiron, hydroxylamine) can help detect and quantify other soil components such as allophane and magnetite.

Received for publication March 25, 1996.

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